In a railway vehicle, although the weight of the vehicle acts on rails through respective wheels, a vertical load acting on each wheel is referred to as “the wheel load.” Moreover, the balance of the wheel loads between the respective wheels varies and, thus, a state where the wheel load on a certain wheel becomes extremely small is referred to as “the decrease in wheel load.”
Meanwhile, since the orbit changes from a plane state into a cant state with the distance at an entrance part into an orbital curve (transition curve part) as shown in FIG. 26, the orbit becomes in a twisted state as seen from the carbody. If a railway vehicle where an existing rigid carbody is supported by bogies via normal suspensions travels through such a curved cant gradually decreasing section, the decrease in wheel load may occur at a front wheel on the outside rail (part 3b shown in FIG. 26) due to the orbital torsion especially during traveling an exit part of the curve.
As the countermeasure, as disclosed, for example, in Patent Documents 1 and 2, devices installed between the carbody and the bogies and for controlling the heights of air springs for supporting the carbody (i.e., the pressures in the air springs) to reduce the occurrence of the decrease in wheel load, have been proposed.
In a freight train, so-called indirect-mounting bogies are normally used, and the carbody is supported by bolsters via side bearers and center pivots. The bolsters are supported by a bogie frame via bolster springs, and the bolster springs support a force in the roll directions. In the freight train, there is a special nature that loads acting on the bogies change a lot between an empty-vehicle state and a loaded state.
If there is initial torsion between supporting points of the front bogie and the rear bogie of the carbody, unbalance may occur mainly in the bolster spring supporting heights of each of the front and rear bogies and, thereby, unbalance may occur in static load on the left and right wheels respectively in the front and rear bogies. This unbalance in the static load on the left and right wheels may promote the decreases in wheel load when traveling through the curve.
As the countermeasure, for example, Patent Documents 3 and 4 have proposed a technique to achieve equalization of the bolster spring supporting heights of each of the front and rear bogies, against the initial torsion of the carbody, by inserting liners in bolster spring supporting parts of the front and rear bogies, respectively.